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61-564: The Edziza Formation ( / ə d ˈ z aɪ z ə / əd-zy-zə ) is a stratigraphic unit of Pleistocene age in northwestern British Columbia , Canada. First described in 1984, the Edziza Formation was mapped as one of several geological formations of the Mount Edziza volcanic complex . It overlies at least four other geological formations of this volcanic complex that differ in age and composition. The main volcanic rock comprising

122-458: A composite volcano , is a conical volcano built up by many alternating layers ( strata ) of hardened lava and tephra . Unlike shield volcanoes , stratovolcanoes are characterized by a steep profile with a summit crater and explosive eruptions. Some have collapsed summit craters called calderas . The lava flowing from stratovolcanoes typically cools and solidifies before spreading far, due to high viscosity . The magma forming this lava

183-747: A 4-inch thick ash layer can weigh 120-200 pounds and can get twice as heavy when wet. Wet ash also poses a risk to electronics due to its conductive nature. Dense clouds of hot volcanic ash can be expelled due to the collapse of an eruptive column , or laterally due to the partial collapse of a volcanic edifice or lava dome during explosive eruptions . These clouds are known as pyroclastic surges and in addition to ash , they contain hot lava , pumice , rock , and volcanic gas . Pyroclastic surges flow at speeds over 50 mph and are at temperatures between 200 °C – 700 °C. These surges can cause major damage to property and people in their path. Lava flows from stratovolcanoes are generally not

244-426: A crudely radial pattern towards the summit crater. They individually range in thickness from a few centimetres to nearly 2 kilometres (1.2 miles) and occur both within the stratovolcano and in the underlying basement rocks. Cupolas occur throughout Tenchen Valley as 3-to-9-metre-wide (9.8-to-29.5-foot) projections through thick pyroclastic deposits and resemble inverted sand castings up to 12 metres (39 feet) high where

305-474: A fast moving mudflow . Lahars are typically about 60% sediment and 40% water. Depending on the abundance of volcanic debris the lahar can be fluid or thick like concrete. Lahars have the strength and speed to flatten structures and cause great bodily harm, gaining speeds up to dozens of kilometers per hour. In the 1985 eruption of Nevado del Ruiz in Colombia , Pyroclastic surges melted snow and ice atop

366-515: A final intermediate composition . When the magma nears the top surface, it pools in a magma chamber within the crust below the stratovolcano. The processes that trigger the final eruption remain a question for further research. Possible mechanisms include: These internal triggers may be modified by external triggers such as sector collapse , earthquakes , or interactions with groundwater . Some of these triggers operate only under limited conditions. For example, sector collapse (where part of

427-484: A formation in another region and a formation may reduce in rank for member or bed as it "pinches out". A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in the case of a marker horizon . A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of

488-422: A prolonged basis. At least three trachyte domes of the Edziza Formation occur on Mount Edziza, all of which were created by the rapid effusion of viscous trachytic lava. The nearly circular Nanook Dome forms the southeastern buttress of the summit crater and is the largest of the three lava domes with a diameter of about three-quarters of a kilometre. It may have been the source for the two upper lava lakes inside

549-739: A result of intraplate volcanism on oceanic islands far from plate boundaries. Examples are Teide in the Canary Islands , and Pico do Fogo in Cape Verde . Stratovolcanoes in the East African Rift include Ol Doinyo Lengai in Tanzania, and Longonot in Kenya. Subduction zone volcanoes form when hydrous minerals are pulled down into the mantle on the slab. These hydrous minerals, such as chlorite and serpentine , release their water into

610-555: A shear cliff across the full width of the Tenchen cirque headwall. The uppermost unit is largely buried under overhanging ice, but it appears to consist of agglutinated volcanic blocks. Exposed in the underlying basement rocks on the deeply eroded eastern flank of Mount Edziza are subvolcanic trachyte cupolas , sills , dikes and irregular intrusive masses linked to the magma plumbing system. Dikes are present as swarms and are usually vertical or subvertical, having been distributed in

671-461: A significant threat to humans or animals because the highly viscous lava moves slowly enough for everyone to evacuate. Most deaths attributed to lava are due to related causes such as explosions and asphyxiation from toxic gas . Lava flows can bury homes and farms in thick volcanic rock which greatly reduces property value. However, not all stratovolcanoes erupt viscous and sticky lava . Nyiragongo , near Lake Kivu in central Africa ,

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732-527: Is a passive release of gas during periods of dormancy. As per the above examples, while eruptions like Mount Unzen have caused deaths and local damage, the impact of the June 1991 eruption of Mount Pinatubo was seen globally. The eruptive columns reached heights of 40 km and dumped 17 megatons of SO 2 into the lower stratosphere . The aerosols that formed from the sulfur dioxide (SO 2 ), carbon dioxide (CO 2 ), and other gases dispersed around

793-582: Is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. A sequence of fossil -bearing sedimentary rocks can be subdivided on the basis of the occurrence of particular fossil taxa . A unit defined in this way is known as a biostratigraphic unit, generally shortened to biozone . The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones. Stratovolcano A stratovolcano , also known as

854-550: Is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in the North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. A supergroup

915-523: Is alkali feldspar which comprises small tabular phenocrysts. Permeating the completely altered material are chalcedony veinlets less than 1 millimetre (0.039 inches) thick which are cut by more extensively distributed veins 1 to 5 centimetres (0.39 to 1.97 inches) wide. Tiny cubes of pyrite occupy many of the larger veins while the surrounding wall rock contains disseminated pyrite and marcasite . The conduit breccia and surrounding rocks were likely altered by intense fumarolic activity that occurred on

976-480: Is also the youngest geological formation of the Mount Edziza volcanic complex involving more than 10 cubic kilometres (2.4 cubic miles) of volcanic material. Basalt flows of the 4.4 million-year-old Nido Formation are overlain by the Edziza Formation on the eastern flank of Mount Edziza between Tenchen Creek and The Pyramid where they overlie a ridge of Mesozoic basement rocks. The Edziza Formation overlies

1037-449: Is an expansion of Kakiddi Creek. Stratigraphically , the Edziza Formation is the fifth youngest unit of the Mount Edziza volcanic complex, having been deposited over the Nido, Pyramid , Ice Peak and Pillow Ridge formations. It has a volume of 18 cubic kilometres (4.3 cubic miles), making it the fifth most voluminous geological formation of the Mount Edziza volcanic complex. The Edziza Formation

1098-549: Is embedded with sparse phenocrysts of anorthoclase and sodic ferrohedenbergite. Edziza trachyte is mineralogically similar to trachyte of the Kakiddi Formation which was erupted 0.3 million years ago during the final stages of the fourth magmatic cycle . However, the Kakiddi trachyte flows travelled along gently sloping valleys for at least 10 kilometres (6.2 miles), suggesting that they were extruded more fluidly than those of

1159-476: Is often felsic , having high to intermediate levels of silica (as in rhyolite , dacite , or andesite ), with lesser amounts of less viscous mafic magma . Extensive felsic lava flows are uncommon, but can travel as far as 8 km (5 mi). The term composite volcano is used because the strata are usually mixed and uneven instead of neat layers. They are among the most common types of volcanoes; more than 700 stratovolcanoes have erupted lava during

1220-571: Is sparsely overlain by the Edziza Formation, but much larger portions of the Edziza Formation overlap with the western and northern edges of Pillow and Tsekone ridges, respectively. Hawaiite and patches of air-fall tephra assigned to the Holocene Big Raven Formation overlie the northern end of the Edziza Formation where they are part of the Desolation Lava Field . The Edziza Formation consists mainly of trachyte that

1281-451: Is the most famous example of a hazardous stratovolcano eruption. It completely smothered the nearby ancient cities of Pompeii and Herculaneum with thick deposits of pyroclastic surges and pumice ranging from 6–7 meters deep. Pompeii had 10,000-20,000 inhabitants at the time of eruption. Mount Vesuvius is recognized as one of the most dangerous of the world's volcanoes, due to its capacity for powerful explosive eruptions coupled with

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1342-450: Is typically between 700 and 1,200 °C (1,300-2,200 °F). Volcanic bombs are masses of unconsolidated rock and lava that are ejected during an eruption. Volcanic bombs are classified as larger than 64mm (2.5 inches). Anything below 64mm is classified as a volcanic block . When erupted Bombs are still molten and partially cool and solidify on their descent. They can form ribbon or oval shapes that can also flatten on impact with

1403-403: Is very dangerous because its magma has an unusually low silica content , making it much less viscous than other stratovolcanoes. Low viscosity lava can generate massive lava fountains , while lava of thicker viscosity can solidify within the vent, creating a volcanic plug . Volcanic plugs can trap gas and create pressure in the magma chamber, resulting in violent eruptions. Lava

1464-800: The Holocene Epoch (the last 11,700 years), and many older, now extinct, stratovolcanoes erupted lava as far back as Archean times. Stratovolcanoes are typically found in subduction zones but they also occur in other geological settings. Two examples of stratovolcanoes famous for catastrophic eruptions are Krakatoa in Indonesia (which erupted in 1883 claiming 36,000 lives) and Mount Vesuvius in Italy (which erupted in 79 A.D killing an estimated 2,000 people). In modern times, Mount St. Helens (1980) in Washington State , US, and Mount Pinatubo (1991) in

1525-636: The Mount Edziza volcanic complex , a group of late Cenozoic volcanic rocks in northwestern British Columbia , Canada. In 1988, Jack Souther mapped the Edziza and Pillow Ridge formations separately and the number of geological formations comprising the Mount Edziza volcanic complex had dropped to 13; the Sheep Track and Kounugu formations had been reassigned as members of the Big Raven and Nido formations, respectively, and are no longer recognized. Nearly all of

1586-573: The Philippines have erupted catastrophically, but with fewer deaths. Stratovolcanoes are common at subduction zones , forming chains and clusters along plate tectonic boundaries where an oceanic crust plate is drawn under a continental crust plate (continental arc volcanism, e.g. Cascade Range , Andes , Campania ) or another oceanic crust plate ( island arc volcanism, e.g. Japan , Philippines , Aleutian Islands ). Stratovolcanoes also occur in some other geological settings, for example as

1647-569: The Tahltan language , referring to the deep volcanic ash deposits or pumice -like sand covering large portions of the Big Raven Plateau around Mount Edziza . According to David Stevenson of University of Victoria 's Anthropology Department, "sand" or "dust" is instead translated as "kutlves" in the Tahltan language. An explanation listed in the BC Parks brochure is that Edziza means "cinders" in

1708-891: The magma is too viscous to allow easy escape of volcanic gases . As a consequence, the tremendous internal pressures of the trapped volcanic gases remain and intermingle in the pasty magma . Following the breaching of the vent and the opening of the crater, the magma degasses explosively. The magma and gases blast out with high speed and full force. Since 1600 CE , nearly 300,000 people have been killed by volcanic eruptions . Most deaths were caused by pyroclastic flows and lahars , deadly hazards that often accompany explosive eruptions of subduction-zone stratovolcanoes. Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fine ash , fragmented lava , and superheated gases that can travel at speeds over 150 km/h (90 mph). Around 30,000 people were killed by pyroclastic flows during

1769-443: The mantle which decreases its melting point by 60 to 100 °C. The release of water from hydrated minerals is termed " dewatering ", and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. This allows the mantle to partially melt and generate magma . This is called flux melting . The magma then rises through the crust , incorporating silica-rich crustal rock, leading to

1830-607: The 1902 eruption of Mount Pelée on the island of Martinique in the Caribbean . During March and April 1982, El Chichón in the State of Chiapas in southeastern Mexico , erupted 3 times, causing the worst volcanic disaster in that country's history and killied more than 2,000 people in pyroclastic flows . Two Decade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On 15 June, Mount Pinatubo erupted and caused an ash cloud to shoot 40 km (25 mi) into

1891-481: The 1982 eruption of Galunggung in Java , British Airways Flight 9 flew into the ash cloud, causing it to sustain temporary engine failure and structural damage. Although no crashes have happened due to ash, more than 60, mostly commercial aircraft , have been damaged. Some of these incidents resulted in emergency landings. Ashfalls are a threat to health when inhaled and are also a threat to property. A square yard of

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1952-585: The 5,321 m (17,457 ft) high Andean volcano. The ensuing lahar killed 25,000 people and flooded the city of Armero and nearby settlements. As a volcano forms, several different gases mix with magma in the volcanic chamber. During an eruption the gases are then released into the atmosphere which can lead to toxic human exposure. The most abundant of these gases is H 2 O ( water ) followed by CO 2 ( carbon dioxide ), SO 2 ( sulfur dioxide ), H 2 S ( hydrogen sulfide ), and HF ( hydrogen fluoride ). If at concentrations of more than 3% in

2013-419: The Edziza Formation is trachyte which was deposited by volcanic eruptions at the end of the third magmatic cycle of the Mount Edziza volcanic complex 0.9 million years ago. Trachyte of the Edziza Formation is in the form of lava flows and pyroclastic rocks that comprise the central stratovolcano of Mount Edziza , as well as lava domes on its summit and flanks. At least four lava lakes ponded inside

2074-559: The Edziza Formation is located on the east-central edge of the Big Raven Plateau, a barren plateau in Cassiar Land District bounded on the west by Mess Valley , on the north by Klastline Valley , on the east by Kakiddi Valley and on the south by Chakima and Walkout valleys, the latter two of which are separated by mountainous terrain . It lies at the northern end of the Mount Edziza volcanic complex which also includes

2135-443: The Edziza Formation. The main feature consisting of Edziza Formation trachyte is the central stratovolcano of Mount Edziza. It comprises explosion breccias, landslide or lahar deposits and thick, steeply-dipping flows that were erupted as highly viscous lava. These rocks are well-exposed in 850-metre-high (2,790-foot) cliffs on the eastern flank of Mount Edziza where they occur along the north side of Tenchen Valley. This section of

2196-462: The Glacier Dome eruption is exposed for 2 kilometres (1.2 miles) north of Pyramid Creek; its distal end thins to about 15 metres (49 feet). South of Pillow Ridge on the northwestern flank of Mount Edziza is a rounded pyroclastic cone with a small central crater that probably formed during the latter stages of Edziza Formation volcanism . This pile of agglutinated trachyte spatter, pumice and bombs

2257-541: The Tahltan language. Another explanation proposed by Canadian volcanologist Jack Souther is that Edziza is a corruption of Edzerza, the name of a local Tahltan family. The Edziza Formation was first described by Jack Souther, Richard Lee Armstrong and J. Harakal in 1984 who grouped it together with the Pillow Ridge Formation in their descriptions and mapping. It was mapped as one of 15 geological formations of

2318-816: The air, when breathed in CO 2 can cause dizziness and difficulty breathing. At more than 15% concentration CO 2 causes death. CO 2 can settle into depressions in the land, leading to deadly, odorless pockets of gas. SO 2 classified as a respiratory, skin, and eye irritant if come into contact with. It is known for its pungent egg smell and role in ozone depletion and has the potential to cause acid rain downwind of an eruption. H 2 S has an even stronger odor than SO 2 as well as being even more toxic. Exposure for less than an hour at concentrations of over 500 ppm causes death. HF and similar species can coat ash particles and once deposited can poison soil and water. Gases are also emitted during volcanic degassing, which

2379-741: The air. It produced large pyroclastic surges and lahar floods that caused a lot of damage to the surrounding area. Pinatubo , located in Central Luzon just 90 km (56 mi) west-northwest of Manila , had been dormant for six centuries before the 1991 eruption. This eruption was one of the 2nd largest in the 20th century. It produced a large volcanic ash cloud that affected global temperatures, lowering them in areas as much as .5 °C. The volcanic ash cloud consisted of 22 million tons of SO 2 which combined with water droplets to create sulfuric acid . In 1991 Japan's Unzen Volcano also erupted, after 200 years of inactivity. It's located on

2440-415: The breccia has eroded away. The Tenchen cirque headwall exposes dikes, sills and breccias within the central conduit that have been intensely altered by hydrothermal solutions, resulting in the rocks being bright yellow and ochre -weathered. Complete alteration occurs in the middle of the altered zone where rock has been reconstituted to a pure white, amorphous material; the only original mineral present

2501-479: The contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%". Sequences of sedimentary and volcanic rocks are subdivided on the basis of their shared or associated lithology . Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank , higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank,

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2562-520: The crater where the eastern side of the summit ridge has been breached by active cirques. They overlie breccia of the central conduit and are in the form of at least four cooling units. Trachyte with well-developed columnar jointing forms the lower two units, each of which is about 30 metres (98 feet) thick. The two upper units consist of lighter green trachyte; the lower unit has a thickness of nearly 90 metres (300 feet). Well-developed, vertical and rectangularly jointed trachyte forms this unit and comprises

2623-540: The flank of a volcano collapses in a massive landslide) can only trigger the eruption of a very shallow magma chamber . Magma differentiation and thermal expansion also are ineffective as triggers for eruptions from deep magma chambers . In recorded history , explosive eruptions at subduction zone ( convergent-boundary ) volcanoes have posed the greatest hazard to civilizations. Subduction-zone stratovolcanoes, such as Mount St. Helens , Mount Etna and Mount Pinatubo , typically erupt with explosive force because

2684-437: The formation. A member need not be mappable at the same scale as a formation. Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although the boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area. A group

2745-478: The ground. Volcanic Bombs are associated with Strombolian and Vulcanian eruptions and basaltic lava . Ejection velocities ranging from 200 to 400 m/s have been recorded causing volcanic bombs to be destructive. Lahars (from a Javanese term for volcanic mudflows) are a mixture of volcanic debris and water. Lahars can result from heavy rainfall during or before the eruption or interaction with ice and snow. Meltwater mixes with volcanic debris causing

2806-458: The high population density of the surrounding Metropolitan Naples area (totaling about 3.6 million inhabitants). In addition to potentially affecting the climate, volcanic ash clouds from explosive eruptions pose a serious hazard to aviation . Volcanic ash clouds consist of ash which is made of silt or sand sized pieces of rock, mineral, volcanic glass . Ash grains are jagged, abrasive, and don't dissolve in water. For example, during

2867-500: The island of Kyushu about 40 km (25 mi) east of Nagasaki . Beginning in June, a newly formed lava dome repeatedly collapsed. This generated a pyroclastic flow that flowed down the mountain's slopes at speeds as high as 200 km/h (120 mph). The 1991 eruption of Mount Unzen was one of the worst volcanic disasters in Japan's history, once killing more than 15,000 people in 1792. The eruption of Mount Vesuvius in 79 AD

2928-415: The main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by geological surveys . Units of formation or higher rank are usually named for the unit's type location , and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to

2989-460: The size of Mount Pinatubo affected the weather for a few years; with warmer winters and cooler summers observed. A similar phenomenon occurred in the April 1815, the eruption of Mount Tambora on Sumbawa island in Indonesia . The Mount Tambora eruption is recognized as the most powerful eruption in recorded history. Its eruption cloud lowered global temperatures as much as 0.4 to 0.7 °C. In

3050-554: The smaller Arctic Lake and Kitsu plateaus to the south. This complex of shield volcanoes , stratovolcanoes , lava domes , calderas and cinder cones forms a broad, intermontane plateau at the eastern edge of the Tahltan Highland , a southeast-trending upland area extending along the western side of the Stikine Plateau . Small portions of the Edziza Formation occur north, northwest and southwest of Nuttlude Lake which

3111-429: The southern end of the ridge are the highest and consist of greenish grey, sparsely porphyritic trachyte. They comprise well-formed, small diameter rock columns that rise nearly vertically for more than 90 metres (300 feet) above the ice cap. Nunataks elsewhere on the summit ridge are more subdued, consisting of pyroclastic debris that has been glacially reworked. The remnants of several lava lakes are exposed inside

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3172-409: The steep Tenchen cirque headwall are large volcanic blocks and small breccia fragments within the central conduit that accompanied with collapse of a narrower summit with a much smaller crater . The collapse may have been caused by a violent, climactic eruption, effusion of the Kakiddi lava flows or the outpouring of voluminous lava during dome formation. Prior to collapse, the summit of Mount Edziza

3233-527: The stratovolcano contains a lower assemblage of chaotic explosion breccias and an upper assemblage of lava that overlies the breccias. Lava of the upper assemblage is highly irregular or lenticular in cross section , but it contains individual cooling units as much as 150 metres (490 feet) thick. The lower assemblage encloses up to 120-metre-long (390-foot) and 15-metre-thick (49-foot) lavas that occur as relatively small, crudely-jointed, irregular masses and discontinuous lenses. Extremely coarse breccias occur in

3294-508: The summit crater which cooled into four rock units that are exposed in the breached eastern crater rim. These lava lakes overlie rock fragments inside the central volcanic conduit which accompanied with collapse of a narrower and higher summit. A number of explanations have been made regarding the origin of the name Edziza. A 1927 report by J. Davidson of the British Columbia Land Surveyors claims that Edziza means "sand" in

3355-483: The summit crater as they consist of trachyte similar to that of Nanook Dome. Glacier Dome and Triangle Dome are two elliptical masses of trachyte with concentric flow layering on the northwestern and western flanks of Mount Edziza, respectively. Triangle Dome displays a pattern of columnar cooling joints that indicates it was formed by volcanic activity in a subglacial environment. A 24-metre-thick (79-foot) trachyte flow that may have issued during an early, fluid phase of

3416-422: The western proximity and contain massive, up to 7.5-metre-wide (25-foot) volcanic blocks that were probably deposited directly from the central conduit during eruption. At the eastern end of Tenchen Valley are more distal breccias that rarely contain volcanic blocks more than 1.5 metres (4.9 feet) wide. Local rounding and stratification of these breccias is indicative of lahar or landslide deposition. Exposed in

3477-423: The westernmost portion of Sphinx Dome , a rhyolite dome of the 1.1 million-year-old Pyramid Formation; two thick remnants of the Edziza Formation also occur on the steep northern and southern flanks of this dome. Basalt flows of the roughly one million-year-old Ice Peak Formation are overlain by the Edziza Formation which overlaps with the northern flank of Ice Peak. The 0.9 million-year-old Pillow Ridge Formation

3538-412: The world. The SO 2 in this cloud combined with water (both of volcanic and atmospheric origin) and formed sulfuric acid , blocking a portion of the sunlight from reaching the troposphere . This caused the global temperature to decrease by about 0.4 °C (0.72 °F) from 1992 to 1993. These aerosols caused the ozone layer to reach the lowest concentrations recorded at that time. An eruption

3599-422: Was at least 610 metres (2,000 feet) higher than its current elevation of 2,786 metres (9,140 feet). The broad, nearly flat summit of Mount Edziza is truncated by a 3-kilometre-wide (1.9-mile) crater that formed after collapse of the original summit. A circular ridge surrounding this ice-filled crater is partially exposed above the ice cap as a discontinuous series of spires and serrated nunataks . Spires forming

3660-421: Was erupted 0.9 million years ago at the end of the third magmatic cycle of the Mount Edziza volcanic complex. Its composition straddles near the pantelleritic trachyte/ comenditic trachyte boundary and is in the form of explosion breccias , lava flows and lava domes. The trachyte contains a matrix of sodic plagioclase alkali feldspar , katophorite , arfvedsonite , aenigmatite and opaque oxides that

3721-480: Was the source of at least two separate lobes of lava that flowed onto the adjacent plateau surface. The northwesterly lobe partially encircles Tsekone Ridge and borders as well as overlies breccia of Pillow Ridge. Extending west of the pyroclastic cone is the second lobe of lava which, together with the northwesterly lobe, is largely overlain by volcanic ash and colluvial material . Stratigraphic unit Units must be mappable and distinct from one another, but

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